Generally, cold cathode fluorescent lamps of this type have been widely used in applications of backlights of liquid crystal displays and so on because it has a longer electrode life and is easily miniaturized as compared with a hot cathode fluorescent lamp using a filament. The cold cathode fluorescent lamp comprises, as generally depicted at symbol 100 in FIG. 13, a fluorescent lamp tube 101 in which a phosphor is applied to an inner surface thereof, a pair of opposing electron emitting electrodes 102, and lead wires 104 electrically connected to the electron emitting electrodes 102, respectively. The fluorescent lamp tube 101 is filled with a gas.
The fluorescent lamp tube 101 used in such a fluorescent lamp is normally formed by a glass tube and the electron emitting electrodes 102 are normally made of a low work function material, such as Ni, Ta, or Zr. Further, as the gas enclosed in the tube 101, a Hg—Ar—Ne mixed gas is normally used.
In manufacturing processes of manufacturing the cold cathode fluorescent lamp 100, the process of cleaning the tube 101 is essential. In the cleaning process of the tube 101, there has conventionally been employed a technique of feeding a cleaning liquid in one direction from one open end of the tube toward the other open end under a constant pressure, i.e. under a normal pressure.
When the cold cathode fluorescent lamp having the foregoing structure and manufactured by the foregoing technique is used in a liquid crystal display, there is a tendency that a cold cathode fluorescent lamp with a longer lifetime and a higher luminance is required following the spread of the liquid crystal displays.
In order to form the high-luminance cold cathode fluorescent lamp, it is quite important to reduce a cathode voltage drop that is generated near an electrode portion. Further, in order to reduce the cathode voltage drop, there has been widely adopted a hollow cathode structure that confines glow discharge inside a tubular electrode.
In order to further reduce the cathode voltage drop by the use of the hollow cathode, a method may be carried out which includes a simple step of applying a R2O3 type electron emission material to an inner surface of the hollow cathode to reduce an effective work function of the electrode, thereby reducing the cathode voltage drop, as disclosed in Japanese Patent No. 3107743 specification (hereinafter referred to as Reference Document 1).
However, only by applying the electron emission material to the inner surface of the hollow cathode as shown in Reference Document 1, since the thermal conductivity of the electrode material is poor, La2O3 or the like being an electron emission substance evaporates to reduce the electrode life.
According to researches of the present inventors, when the tubular hollow cathode was employed, a phenomenon was observed at the start of discharge that an electric field was concentrated to an open end portion of the tube so that the electrode was sputtered. It has been found that the life of the electrode is shortened as a result of the concentration of the electric field.
Further, it has also been found that since the lead wire for supplying a voltage to the electrode is joined to the electrode by welding, a thermal resistance is generated at the joining interface and thus the heat conduction is not efficiently carried out. Further, since use is made of Ar and Ne each having a poor thermal conductivity as the noble gas components in the filled gas, the heat radiation from the electrode is not efficiently carried out so that the electrode temperature rises to reduce the electrode life.
Reviewing also the manufacturing processes, it has been found that since, in the tube cleaning process among the manufacturing processes, the cleaning liquid is delivered in the single direction and further under the constant pressure, the inside of the thin and long tube cannot be sufficiently cleaned to thereby cause a problem of adhesion failure and uneven application of the phosphor, which also reduces the life of the cold cathode fluorescent lamp.
Further, it has also been found that moisture and oxygen remaining inside the tube reduce the electrode life. With respect to the residual moisture, it has been found that there is a problem in a drying method after the cleaning. With respect to the residual oxygen, it has been found that there is a problem in an exhaust method at the time of seal-cutting the tube.
In a drying method after the cleaning, moisture inside the tube is desorbed by raising a temperature in the atmosphere. However, a problem has occurred wherein the atmospheric components enter the tube when the drying is finished and the tube is cooled, so that moisture in the atmosphere adsorbs again inside the tube.
With respect to the exhaust method at the time of the seal cutting, it has been found that since the tube is long, a pressure difference occurs inside the tube during exhausting so that the gas components inside the tube are not completely exhausted. Further, a problem has occurred wherein the components on the exhaust side of an exhaust pump are diffused back to the inside of the tube so that oxygen remains.
Therefore, it is an object of this invention to provide a fluorescent lamp, particularly a cold cathode fluorescent lamp, that can improve the light emission efficiency by improving the electron emission efficiency and that has a long lifetime.
It is another object of this invention to provide a fluorescent lamp manufacturing method that can achieve an increase in lifetime of a fluorescent lamp, an improvement in luminance, and uniformization of luminance.
It is a specific object of this invention to improve a tube cleaning process in the manufacture of a fluorescent lamp.
It is another specific object of this invention to improve a method and apparatus for drying the inside of a cold cathode lamp tube in the manufacture of a fluorescent lamp, thereby improving the life of an electrode.
It is still another specific object of this invention to improve a method of exhausting a gas inside a cold cathode lamp tube in the manufacture of a fluorescent lamp, thereby improving the life of an electrode.